This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
An electron amplifier structure or an electron multiplier may be used as a component in a detector system to detect low levels of electrons, ions, or photons, and provide an amplified response via a plurality of secondary electron emissions. Examples of electron amplifier structures include a channeltron (single channel tube) and a microchannel plate (MCP).
An MCP is comprised of an array of narrow pores in a flat plate that permeate from the front surface of the plate to the back surface of the plate. For example, the MCP may be a two dimensional array comprised of millions of 5-20 μm diameter pores. A high voltage is applied across the plate such that the back surface is typically at 1000 V higher potential than the front surface. An electron enters the front of an MCP into a channel, and impinges on the channel wall causing secondary electron emissions to be produced by an emissive layer on the channel surface. These secondary electrons are accelerated towards the back of the plate by the high voltage bias and impact on the channel wall to produce additional secondary electrons resulting in a cascading increase in electrons along the length of the MCP channel that exit the opposite end of the channel. Since the MCP pores operate independently, a spatial pattern of electrons incident on the front surface will be preserved so that the back surface emits the same pattern but greatly amplified. In this way, the MCP may be used in imaging applications. Two or more MCPs may be placed in series to provide multiple stages of amplification. Various detectors may be located downstream of the MCP to detect and record the exiting electrons. A photocathode located upstream of the MCP can be used to convert photons incident on the front surface of the photocathode into electrons which exit the back surface and impinge on the MCP to yield a photodetector. MCP-based photodetectors can provide excellent temporal and spatial resolution, very high gain, and significantly low background signal with usability inside magnetic fields as well as cryogenic temperatures with extended life time.
Electron amplifier structures, in particular, MCP detectors have numerous applications, including use in night vision technology, medical imaging devices, homeland security, and particle detectors for use in laboratories and high-energy physics or nuclear physics installations. Neutron detection is the effective detection of neutrons entering in a well-positioned detector. Most neutron detection techniques rely on the detection of the energetic particles and photons emitted upon the absorption of neutrons by these nuclides. Existing neutron detection techniques include solid state converter films and gas-filled converters, which upon neutron absorption emit photons or charged particles that are subsequently detected by a readout device, such as charge coupled devices (CCDs); foil activation followed by the post processing readout with imaging plates; scintillating fibers or plates; and superheated liquid detectors. The currently most favored neutron detectors are based on pressurized gaseous (such as 3He, LiF or BF3) in tubes. Higher detection efficiency can always be achieved with enlarged detection volume, for example, larger 3He or BF3 counter and thicker lithium glass. But larger volumes degrade spatial resolution caused by possibly longer ionization tracks or scattering related photon distribution. A compromise between high efficiency and fine spatial resolution must be reached. Such large volume detectors are very expensive and of limited supply for widespread application due partly to the world-wide shortage of 3He. To acquire fine spatial resolution, the capability of localizing charged particle within a short range is necessary. Gas is excluded due to its poor stopping power for energetic charged particles and solid or liquid material is the better choice. Furthermore, pressurized gas systems may present an explosion hazard unacceptable in certain transport situations.
The compact geometry of MCP channels is an ideal structure for fine spatial resolution. realization. Thermal neutron detectors using MCPs exist, but they do not provide the desired neutron detection efficiency or gamma rejection over large areas. Consequently, these devices could never be scaled to the sizes and quantities needed for applications such as homeland security and treaty verification. Since traditional MCPs have been manufactured with lead glass and are expensive, many neutron detectors use MCPs in photodetectors to detect the light from a neutron-sensitive scintillator and do not capitalize on the cost and efficiency advantages of neutron-10B interactions in the MCP bulk glass material. Neutron detection in the bulk material of MCPs has been demonstrated in small (40 mm diameter) detectors by NOVA Scientific, which manufactures and sells small pore 10B- and Gadolinium-doped neutron-absorbing MCPs incorporated in sealed devices. NOVA Scientific has successfully doped 10B and 157Gd into MCP glass. However, doping concentration conversion material is limited by the production process of MCP glass, which leads to the low density of neutron sensitive nuclides that hinders the improvement of detection efficiency. However, these types of specialized Gd incorporated MCPs are very expensive and may also have limited supply. Furthermore, large area Gd incorporated MCPs suitable for very compact neutron detectors have not yet demonstrated and will be extremely expensive to scale-up to large areas with conventional manufacturing and packaging techniques. In addition the detection of various type neutrons such as cold, thermal, fast, epithermal and relativistic need a different type of neutron sensitive element incorporation in to the MCP glass which is not straightforward and may be very expensive.
A need exists for improved technology, including a fabrication method for electron amplifier structures or devices that improve detection efficiency, enhance device gain, or prolong an operating life of the device.